1. Field of Industrial Applicability
This invention relates to an autoshut-off mechanism for controlling the torque in the shaft of a torque wrench.
2. Prior Art
A torque wrench using an air motor as a power source is provided with a shut-off mechanism adapted to stop the rotation of the motor when a bolt or nut has been seated by a predetermined value of torque. The autoshut-off mechanism which develops this function operates on the principle of detecting the stoppage or reversal of rotation of the rotor by means of an inertial body connected to the rotor, the movement of said inertial body opening an air feed valve to feed air to an air timer, then shutting off the feeding of air to the air motor after the lapse of given time after the start of seating.
Further, Japanese Utility Model Application Disclosure No. 40078/1991 discloses an autoshut-off device for a torque wrench adapted to intermittently transmit the rotational output from an air motor and including a pulse generating mechanism which produces an oil pressure pulse each time the torque is imparted to the main shaft, said autoshut-off device being characterized in that the oil pressure in the high pressure producing side of said mechanism acts on a relief valve, so that the latter is opened when said oil pressure exceeds a set pressure set by the spring pressure of said relief valve, the then--prevailing oil pressure being used to stop the rotation of the air motor.
In the two prior art examples described above, the former, though capable of providing some degree of torque accuracy, is not so designed as to merit for the produced torque to actuate the autoshut-off device by said generated torque; therefore, there has been a problem that the torque accuracy is readily influenced by the performance of the torque wrench itself or by the operating conditions.
In the latter, since an oil pressure pulse can be generated only to a very short time, there is a drawback that the oil pressure often fails to be transmitted to a bolt or nut to serve as a tightening force. Further, since the air motor will be stopped if the oil pressure in the high pressure producing side becomes higher than the spring pressure of the relief valve, only the lower limit of the generated torque is monitored; there has been a problem that a variation in the performance of machine parts, such as springs, unstabilizes the operating accuracy.